Y axis is fine, take an example from the cube.json file:
{
"parent": "block/block",
"elements": [
{ "from": [ 0, 0, 0 ],
"to": [ 16, 16, 16 ],
"faces": {
"down": { "texture": "#down", "cullface": "down" },
"up": { "texture": "#up", "cullface": "up" },
"north": { "texture": "#north", "cullface": "north" },
"south": { "texture": "#south", "cullface": "south" },
"west": { "texture": "#west", "cullface": "west" },
"east": { "texture": "#east", "cullface": "east" }
}
}
]
}
On the Y axis (in green) we can see that the block is going from y=0 to y=16
So I conclude that 0 is the bottom and 16 is the top of a typical minecraft block.